1. Field of the Invention
This invention relates to a control system, and more particularly, to an improved high accuracy numerical control system.
2. Description of the Prior Art
When a machine tool is controlled by a numerical control system, there are applications where it is necessary to control the machine tool to achieve the greatest accuracy possible. In a numerical control system using a resolver feedback, the accuracy of the system is related to the amount of detected movement of the tool or member that will cause a shift in the phase of an electrical output signal from the resolver over one electrical cycle, wherein the phase of the output signal is indicative of the actual position of the member.
In a standard position loop numerical control system, the output signal from the resolver is applied to a waveshaper to produce a square wave feedback signal having cyclical transitions, wherein the period between successive transitions is equal to the previously referred to electrical cycle. This feedback signal is compared with a command signal in a phase discriminator. The command signal is also a square wave signal having cyclical transitions, wherein the phase of these transitions is indicative of the commanded (desired) movement of the member. The difference in phase between the command signal and the feedback signal is detected in the phase discriminator, and the detected error signal is used to drive a servo amplifier and motor to cause the member to move toward the desired position. The period between successive cyclical transitions in the command signal is also equal to the previously referred to electrical cycle. The phase of the electrical cycle defined by successive cyclical transitions in the command signal shifts by receiving incremental command pulses, wherein one thousand command pulses cause the phase of the command signal to shift by one electrical cycle. Thus, if the shift in phase of the output signal from the waveshaper through one electrical cycle would represent 0.1" in movement of the member, then the shift in phase of the command signal through one electrical cycle would represent 0.1" in desired movement of the member, and each incremental command pulse represents 0.0001" in commanded movement. This 0.0001" is the smallest increment in commanded movement and is referred to as the command resolution of the system.
To increase the system accuracy and also the fineness of the command resolution of the system, it would be necessary to increase the sensitivity of the resolver by providing that the movement of the member, which is necessary to cause the output signal from the resolver to shift one electrical cycle, is smaller. To maximize the accuracy of the system, a gearing mechanism can be interposed between the motor output and the resolver. The ratio of the gearing mechanism would depend upon the maximum speed that the resolver can be driven. Under the current state of the technology, if, for example, the resolver could be driven to its limit, determined by a gear ratio of 5:1, the amount that the member must move to cause the phase of the output of the resolver to shift one electrical cycle would be one-fifth as great as before or 0.02". Thus, each increment of commanded movement (i.e., command resolution) would be equal to 0.00002" or 20 microns. Under these circumstances, the system would achieve a finer resolution and higher accuracy. However, at the same time, in view of the fact that each increment of commanded movement for the system is equal to 0.00002" or 20 microns, the maximum commanded movement and feed or jog rate that the system can achieve may be lower than desired when large amounts of metal removal in the shortest time is most important.